GEOLOGY and show that the sands are of salt water origin and of the age of the Lower Barton Beds of Hampshire. The soil over the Bagshot and Barton Beds is very poor, and the ground is to a great extent uncultivated and covered with heather and fir plantations. It forms however healthy residential tracts. The soil of the Bracklesham Beds is slightly better, and fine trees grow on it at Swinley and in many other places. Springs are thrown out at the bottom of the Barton Beds and a fair supply of water may often be obtained in wells sunk to that horizon; the upper part of the sands seldom contains any water. DRIFT AND SUPERFICIAL DEPOSITS The Bagshot series is the most recent formation in Berkshire which can with certainty be stated to have been deposited in sea water. At some time after Upper Bagshot and Lower Barton date, elevation of this part of England took place and Berkshire became dry land, and the oldest record of dry-land condition is probably the clay with flints. THE CLAY WITH FLINTS covers a large part of the Chalk district. It consists of clay, loam and earth full of flints, which retain their original irregular shape and have not been rolled or waterworn. The deposit is of the most variable thickness up to about 20 feet, often filling pipes or hollows in the Chalk. It is believed to be largely due to the dissolution of the Chalk near the surface of the ground by the action of water which percolates through it, the water carrying away in solution the carbonate of lime and any other easily soluble minerals and leaving the insoluble residue, i.e. flints and earthy or loamy material, with which is often mixed clay and flint pebbles, the relics of Eocene beds which lay on the surface of the Chalk. 1 The irregularity of the deposit and the pipes in the Chalk are due to the irregular course underground taken by the percolating water. Sometimes these hollows in the Chalk are of considerable size and form what are known as ' swallow holes,' since they swallow up the water which flows into them. Mr. Whitaker explains that the swallow holes are often due to streams which, rising in the higher ground, flow down an escarpment of Eocene beds until they reach the pervious and jointed Chalk, the water flowing into which forms in time a swallow hole through the chemical action of the carbonic acid which it contains, assisted by the mechanical action of the water itself ; and the presence of swallow holes at a dis- tance from Eocene clays is probably an indication of their former extent in comparatively recent geological times. Some of the best timber in the county grows upon the clay with flints, and good crops are often found on it in spite of the stony ground. It is a sub-aerial deposit the sea has had no part in its formation and as the process must have been very slow, its thickness and great 1 See W. Whitaker, ' Geology of London,' Geol. Survey (1889), i. 281. 19
